The present invention relates to a process for the preparation of polyurethane elastomers in which 1,4-monoanhydroerythritol and/or 1,4-monoanhydrothreitol is (are) used as bifunctional chain lengthening agents.
Monoanhydro-tetritols may be obtained in excellent yields by processes known in the literature. In one such process, a tetritol is heated in the presence of heterogeneous, acid ion-exchange resins to temperatures below 160.degree. C. 1,4-monoanhydroerythritol may be obtained in 94 to 98% yield from solvent-free erythritol at 140.degree. C. under vacuum (see F. H. Otey et al, J. Org. Chem., 26, 1673 (1961)). This monoanhydrotetritol has been suggested as a possible starter diol for epoxides in the production of hydroxypolyether diols which are conventional starting materials for polyurethanes.
German Offenlegungsschrift No. 3,041,673 (European Patent Application 52,295) describes a process for the preparation of monoanhydrotetritols or mono- and/or bis-anhydrohexitols by a solvent-free reaction of tetritols or hexitols in the presence of ion-exchange resins at temperatures below 160.degree. C., optionally under vacuum. This disclosure also describes the use of such anhydrous sugar alcohols as starting materials for chemical syntheses of materials such as polyurethanes.
The publication by H. U. Woelk in Nachr. Chem. Tech. Lab. 30, 204-206 (1982) also describes the use of cyclic diols (e.g., anhydrous compounds of sorbitol) as interesting components for the synthesis of polyesters and polyurethanes.
The use of short chain diols as chain lengthening agents for the production of polyurethane plastics is well known. Certain combinations of isocyanates and chain lengthening agents, as described, for example, in Kunststoff-Handbuch, Volume VII, Vieweg-Hochtlen, Carl-Hanser-Verlag, Munich, 1966, on pages 206 et seq, have proven to be particularly suitable for the production of polyurethanes with good physical properties. Such combinations include the 1,5-diisocyanatonaphthalene/1,4-butanediol system and the 4,4'-diisocyanato-diphenylmethane/hydroquinone-(2-hydroxyethyl)-ether system. The use of 2,3-butanediol as chain lengthening agent in conjunction with 1,5-diisocyanatonaphthalene is also known to produce high quality polymer products.
The high reactivity of conventional chain lengthening agents results in very short pot lives, especially in systems containing a high proportion of hard segments (high diisocyanate content in the starting mixture). Consequently, systems in which such chain lengthening agents are employed are often difficult to work up into shaped products. In addition, the large amount of heat evolved where the reaction is too rapid may result in a finished, shaped product which is not completely homogeneous. A chain lengthening agent may thus adversely affect the quality of the product.
These disadvantages may be overcome to a large extent by use of 1,4-3,6-dianhydro-hexitols as chain lengthening agents (see German Offenlegungsschrift No. 3,111,093). If the typical chain lengthening agents (e.g., butanediol) are completely replaced by 1,4-3,6-dianhydro-hexitols in casting elastomer systems based on naphthylene-1,5-diisocyanate, the polyurethanes obtained are markedly superior in some of their characteristics, such as tear propagation resistance and structural strength. However, molded polyurethane products prepared with 1,4-3,6-dianhydro-hexitols as chain lengthening agents have a relatively high shrinkage and are difficult to produce in the exact dimensions wanted.